The present invention relates to a measurement apparatus and a method which are applicable to blade tip clearance measurements of blades actually rotating in aircraft jet engine compressors or turbines. The present invention is also applicable to blade tip clearance measurements in gas turbines for power generation, gas turbines for automobiles and other vehicles, and other industrial turbo mechanisms.
FIG. 1 shows a conventional apparatus (see, for example, Barranger, J. P. and Ford, M. J., Journal of Engineering for Power, April. 1981, Vol. 103, pp. 457 -460). In the conventional laser-optical clearance measurement apparatus shown in FIG. 1, a laser beam generated at a laser generator 1 is led to an optical tube 3 via an optical fiber cable 2 for illumination and goes through a prism 4 and is reflected at the tip of a moving blade 5. The reflected light goes through the prism 4 again and enters an image processor 7 via an optical fiber cable 6 for receiving the reflected beam. Changes in the moving blade tip position are displayed on the image processor as the shift of the received beam to a position indicated by a broken line in this figure.
The conventional apparatus has the following problems: (1) Since the prism is located close to major gas flow, combustion products in this flow tend to attach to it; (2) Because the cable for receiving the reflected light beam has to be a bundle of several million fibers so that the position of the beam is captured as an image, it is breakable and expensive; and (3) The image processor contains an image intensifier which is, beng a vacuum tube, susceptible to vibrations and shocks and also expensive. Therefore, although these apparatuses can be used for short testing on the ground, at present they cannot practically be attached to working engines and used for their feedback control. Furthermore, there exist some space problems with large tube diameters associated with the asymmetry of the optical system around its optical axis.